This application claims priority of Japanese Application No. 2002-198949 filed on Jul. 8, 2002 and entitled xe2x80x9cMethod of Controlling Refrigeration Cyclexe2x80x9d.
(1) Field of the Invention
This invention relates to a method of controlling a refrigeration cycle, and more particularly to a method capable of controlling a refrigeration cycle for an automotive air conditioner in a manner minimizing burden on an engine for driving a variable displacement compressor and at the same time with high accuracy.
(2) Description of the Related Art
In an automotive air conditioner, since a compressor thereof is driven by an engine, operating conditions of the air conditioner are largely influenced by operating conditions of the engine. Inversely, the automotive air conditioner is a burden on the engine, which requires extra energy for driving the same. This burden on the engine can be reduced by controlling the automotive air conditioner such that performance coefficient of a refrigeration cycle is optimized.
For example, a refrigeration cycle described in Japanese Unexamined Patent Publication No. 9-264622 uses, as a compressor, an electric motor-driven type whose rotational speed can be controlled, and as an expansion device, a pressure control valve, thereby providing control such that coefficient of performance of the refrigeration cycle is optimized. More specifically, the pressure control valve allows refrigerant to be filled in a space closed by a diaphragm that actuates a valve element, at a density within a range between a saturated liquid density at a refrigerant temperature of 0xc2x0 C. and a saturated liquid density at a critical point of the refrigerant, with respect to a space volume of the hermetically sealed space with its valve hole closed, and controls the pressure and temperature of the refrigerant on an outlet side of a gas cooler along the optimum control curve on which the coefficient of performance of the refrigeration cycle is optimum. Further, as an expansion device, an electric pressure control valve is employed which is driven by a stepping motor. The electric pressure control valve also has the pressure and temperature of refrigerant on an inlet side thereof controlled along the optimum control curve.
Further, when an automotive vehicle performs standing start or acceleration during operation of an automotive air conditioner, to ensure that necessary engine output torque is obtained for the standing start or acceleration without largely increasing fuel consumption, the refrigeration cycle is controlled such that the displacement of a compressor of the refrigeration cycle, that is, driving torque is instantaneously reduced and progressively restored thereafter.
Further, the power output of an engine is required to be controlled depending on operating conditions of the automotive air conditioner which is a burden on the engine. For instance, during operation of the automotive air conditioner, by taking the driving torque of the compressor into account, engine output torque is controlled such that extra driving torque is produced therefor. The driving torque of the compressor is preset to a fixed value, and when the automotive air conditioner is operated, the engine output torque is controlled such that extra driving torque corresponding to the preset fixed value is produced.
The preset driving torque is sometimes largely different from actual driving torque, resulting in control of the engine output torque in a manner deviated from required torque. To cope with this, in a technique described e.g. in Japanese Unexamined Patent Publication No. 2001-180261, the driving torque of a variable displacement compressor is estimated based on a compressor control signal used for external electrical control of the delivery capacity of the variable displacement compressor, and engine output torque is controlled to a value to which the estimated driving torque is added.
As described above, according to the conventional method of controlling the refrigeration cycle, by controlling the expansion device such that the relationship between the pressure and temperature of refrigerant is along the optimum control curve, the refrigeration cycle can be operated efficiently. In this efficient operating method, however, since control points are always on the optimum control curve, the refrigeration cycle is not always controlled to the highest cooling performance. Therefore, even during the start of the refrigeration cycle, the refrigeration cycle is controlled such that it operates with maximum efficiency, which takes time before the refrigeration cycle reaches a steady operating condition.
Further, when the automotive vehicle performs standing start or acceleration, the driving torque of the compressor is controlled, and in such a case, especially when the displacement of the compressor is being restored after reduction thereof, there is no means for accurate estimation of actual driving torque of the compressor. This makes it impossible to control the refrigeration cycle with high accuracy.
Further, as for the driving torque of a variable displacement compressor, which is required for control of engine output torque, due to the fact that the delivery capacity of the variable displacement compressor is reflected on a differential pressure between two pressure monitoring points set in a refrigerant circulation circuit, the delivery capacity of the variable displacement compressor, that is, the driving torque of the compressor is estimated from a compressor control signal used for setting the differential pressure between the two pressure monitoring points. Actually, it is difficult to accurately estimate the driving torque of the variable displacement compressor from the compressor control signal alone.
The present invention has been made in view of the above points, and an object thereof is to provide a method of controlling a refrigeration cycle such that maximum refrigerating capacity can be educed when the refrigeration cycle is started, the driving torque of a variable displacement compressor can be reduced when an automotive vehicle performs standing start or acceleration, and the refrigeration cycle can be operated with the maximum efficiency in a steady operating condition.
To solve the above problem, the present invention provides a method of controlling a refrigeration cycle of an automotive air conditioner, characterized by estimating a differential pressure between a high pressure-side refrigerant pressure and a low pressure-side refrigerant pressure in the refrigeration cycle and a flow rate of refrigerant flowing therethrough, based on a first external electric signal for controlling the differential pressure between the high pressure-side refrigerant pressure and the low pressure-side refrigerant pressure in the refrigeration cycle or the flow rate of refrigerant flowing therethrough and a second external electric signal for controlling a size of a refrigerant passage in the refrigeration cycle, and estimating a compressor variable displacement ratio and driving torque of a compressor based on the estimated differential pressure and flow rate of refrigerant and an engine rotational speed, and controlling the first external electric signal and the second external electric signal such that refrigerating capacity of the refrigeration cycle is maximized when the automotive air conditioner is started, efficiency of the refrigeration cycle is maximized when the vehicle is in a steady traveling condition, and driving torque of the compressor is reduced when the vehicle is in an accelerating condition.
The above and other objects, features and advantages of the present invention will become apparent from the following description when taken in conjunction with the accompanying drawings which illustrate preferred embodiments of the present invention by way of example.